Electromagnetic transducer and portable communicating device

ABSTRACT

An electromagnetic transducer according to the present invention includes: a first diaphragm disposed so as to be capable of vibration; a second diaphragm disposed in a central portion of the first diaphragm, the second diaphragm being made of a magnetic material; a yoke disposed so as to oppose the first diaphragm; a center pole disposed between the yoke and the first diaphragm; a coil disposed so as to surround the center pole; a first magnet disposed so as to surround the coil; and a second magnet disposed on an opposite side of the first diaphragm from the center pole.

This Application is a U.S. National Phase Application of PCTInternational Application PCT/JP00/03083.

1. Technical Field

The present invention relates to an electroacoustic transducer for usein a portable communication device, e.g., a cellular phone or a pager,for reproducing an alarm sound, a melody, or an audio sound voice,responsive to an incoming call.

2. Background Art

FIGS. 18A and 18B show a plan view and a cross-sectional view,respectively, of a conventional electroacoustic transducer 200 of anelectromagnetic type (hereinafter referred to as an “electromagnetictransducer”). The conventional electromagnetic transducer 200 includes acylindrical housing 107 and a disk-shaped yoke 106 disposed so as tocover the bottom face of the housing 107. A center pole 103, which mayform an integral part of the yoke 106, is provided in a central portionof the yoke 106. A coil 104 is wound around the center pole 103. Spacedfrom the outer periphery of the coil 104 is provided an annular magnet105, with an appropriate interspace maintained between the coil 104 andthe inner periphery of the annular magnet 105 around the entirecircumference thereof. The outer peripheral surface of the magnet 105 isabutted to the inner peripheral surface of the housing 107. An upper endof the housing 107 supports a first diaphragm 100 which is made of anon-magnetic disk so that an appropriate interspace exists between thefirst diaphragm 100 and the magnet 105, the coil 104, and the centerpole 103. In a central portion of the first diaphragm 100, a seconddiaphragm 101 which is made of a magnetic disk is provided so as to beconcentric with the first diaphragm 100.

Now, the operation and effects of the above-described conventionalelectromagnetic transducer 200 will be described. In an initial statewhere no current flows through the coil 104, a magnetic path is formedby the magnet 105, the second diaphragm 101, the center pole 103, andthe yoke 106. As a result, the second diaphragm 101 is attracted towardthe magnet 105 and the center pole 103, up to a point of equilibriumwith the elastic force of the first diaphragm 100. If an alternatingcurrent flows through the coil 104 in this state, an alternatingmagnetic field is generated in the aforementioned magnetic path, so thata driving force is generated on the second diaphragm 101. Such drivingforce generated on the second diaphragm 101 causes the second diaphragm101 to vibrate from its initial state, along with the fixed firstdiaphragm 100, due to an interaction with a attraction force which isgenerated by the magnet 105. This vibration transmits a sound.

A resonance frequency of the electromagnetic transducer 200 having theabove-described structure depends on the deformation of the firstdiaphragm 100 in a state where the elastic force of the first diaphragm100 and the attraction force which is generated on the second diaphragm101 by the magnet 105 are at equilibrium.

FIG. 19 illustrates the relationship between a force-displacement curveof the first diaphragm 100 and the attraction force generated on thesecond diaphragm 101 by the magnet 105. The vertical axis of the graphrepresents the force, whereas the horizontal axis of the graphrepresents the displacement of the first diaphragm 100. As shown in FIG.19, the force-displacement curve of the first diaphragm 100 and theattraction force curve (generated by the magnet 105 on the seconddiaphragm 101) intersect each other at an intersection A. In otherwords, the intersection A shows a point at which the elastic force andthe static attraction are at equilibrium. The resonance frequency isdetermined by the elastic constant of the first diaphragm 100 at theintersection A.

In order to decrease the resonance frequency, it is necessary toincrease the mass of the vibrating system (i.e., the first diaphragm 100and the second diaphragm 101) or decrease the elastic constant of thevibrating system. However, it is undesirable to increase the mass of thevibrating system because it results in a decrease in the efficiency ofthe electromagnetic transducer 200. On the other hand, decreasing theelastic constant of the vibrating system too far would produce aforce-displacement characteristic curve shown by the broken line in FIG.19, which does not intersect the attraction force curve (generated onthe second diaphragm 101 by the magnet 105). As a result, the seconddiaphragm 101 will be attracted, along with the first diaphragm 100,onto a magnetic circuit without establishing equilibrium at anyposition.

In other words, since the elastic constant must be kept within a rangesuch that the elastic constant curve intersects the attraction forcecurve, there is a lower design limit to the resonance frequency.Although it becomes possible to decrease the elastic constant bydecreasing the attraction force as well, this results in a decrease inthe generated driving force, so that a sufficient reproduced soundpressure level cannot be obtained.

DISCLOSURE OF THE INVENTION

An electromagnetic transducer according to the present inventionincludes: a first diaphragm disposed so as to be capable of vibration; asecond diaphragm disposed in a central portion of the first diaphragm,the second diaphragm being made of a magnetic material; a yoke disposedso as to oppose the first diaphragm; a center pole disposed between theyoke and the first diaphragm; a coil disposed so as to surround thecenter pole; a first magnet disposed so as to surround the coil; and asecond magnet disposed on an opposite side of the first diaphragm fromthe center pole.

In one embodiment of the invention, the electromagnetic transducerfurther includes: a first housing for supporting the first diaphragm;and a second housing in which the second magnet is disposed.

In another embodiment of the invention, the second magnet has a diskshape.

In still another embodiment of the invention, the second magnet has anannular shape.

In still another embodiment of the invention, an outer diameter of thesecond magnet is equal to or smaller than an outer diameter of thesecond diaphragm in the case of the second magnet having a disk shape.

In still another embodiment of the invention, an outer diameter of thesecond magnet is equal to or greater than an outer diameter of thesecond diaphragm in the case of the second magnet having an annularshape.

In still another embodiment of the invention, the electromagnetictransducer further includes a third magnet in a central portion of atleast one face of the first diaphragm or the second diaphragm.

In still another embodiment of the invention, the second magnet ismagnetized in the same direction as the first magnet.

In still another embodiment of the invention, the second magnet ismagnetized along a radial direction with respect to an axis through acenter of the center pole.

In still another embodiment of the invention, the second diaphragm has athickness which allows a magnetic saturation to occur when the seconddiaphragm is deflected toward the center pole by a predetermineddistance.

In still another embodiment of the invention, the first diaphragm ismade of a magnetic material.

In still another embodiment of the invention, the first diaphragm ismade of a non-magnetic material.

In still another embodiment of the invention, the electromagnetictransducer further includes a first magnetic plate provided between thefirst magnet and the first diaphragm.

In still another embodiment of the invention, the first magnetic platehas an annular shape.

In still another embodiment of the invention, the electromagnetictransducer further includes a second magnetic plate disposed on thesecond magnet.

In still another embodiment of the invention, the second magnetic platehas a disk shape.

In still another embodiment of the invention, the second magnetic platehas an annular shape.

In still another embodiment of the invention, the first diaphragm isshaped so as to provide non-linear displacement characteristics forcanceling non-linearity of a driving force generated on the seconddiaphragm.

In still another embodiment of the invention, there is a substantiallylinear relationship between a resultant of a first attraction force anda second attraction force and a distance between the second diaphragmand the center pole; wherein the first attraction force is a attractionforce generated on the second diaphragm by a magnetic circuit includingthe first magnet, the center pole, and the yoke, and the secondattraction force is a attraction force generated on the second diaphragmby the second magnet.

In still another embodiment of the invention, the first diaphragm isaffixed by being adhered to the first housing.

In still another embodiment of the invention, the first diaphragm isaffixed by being interposed between the first housing and the secondhousing.

In still another embodiment of the invention, the second housing is acover for protecting the first diaphragm and the second diaphragm.

In another aspect of the invention, there is provided a portablecommunication device including any one of the aforementionedelectromagnetic transducers.

In one embodiment of the invention, the portable communication devicefurther includes a third housing having a sound hole therein, whereinthe electromagnetic transducer is disposed so that the first diaphragmand the second diaphragm oppose the sound hole.

In another embodiment of the invention, the second magnet is disposed inthe third housing.

Thus, the invention described herein makes possible the advantage ofproviding an electromagnetic transducer which is capable of reproducinglow-frequency ranges without necessitating a change in the size of thefirst magnet, or the first and second diaphragms, and which is capableof reproducing a sound at a high level and low distortion by virtue ofan increased driving force.

This and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a second housing of an electromagnetictransducer 1000 according to Example 1 of the present invention.

FIG. 1B is a cross-sectional view of the electromagnetic transducer 1000according to Example 1 of the present invention.

FIG. 1C is a plan view of a second magnet in the electromagnetictransducer 1000 according to Example 1 of the present invention.

FIG. 2 is a magnetic flux vector diagram of the electromagnetictransducer 1000 according to Example 1 of the present invention.

FIG. 3 is a graph illustrating the relationship among the outer diameterof the second magnet, attraction force, and driving force in theelectromagnetic transducer 1000 according to Example 1 of the presentinvention.

FIG. 4 is a cross-sectional view of an electromagnetic transducer 2000according to Example 2 of the present invention.

FIG. 5 is a cross-sectional view of an electromagnetic transducer 3000according to Example 3 of the present invention.

FIG. 6 is a magnetic flux vector diagram of the electromagnetictransducer 3000 according to Example 3 of the present invention.

FIG. 7 is a cross-sectional view of an electromagnetic transducer 4000according to Example 4 of the present invention.

FIG. 8 shows the force-displacement characteristic curve of a firstdiaphragm in the electromagnetic transducer 4000 according to Example 4of the present invention.

FIG. 9A is a plan view of an electromagnetic transducer 5000 accordingto Example 5 of the present invention.

FIG. 9B is a cross-sectional view of a second magnet in theelectromagnetic transducer 5000 according to Example 5 of the presentinvention.

FIG. 10 is a magnetic flux vector diagram of the electromagnetictransducer 5000 according to Example 5 of the present invention.

FIG. 11 is a graph illustrating attraction forces generated on a seconddiaphragm in the electromagnetic transducer 5000 according to Example 5of the present invention.

FIG. 12 is a graph illustrating driving forces generated on a seconddiaphragm in the electromagnetic transducer 5000 according to Example 5of the present invention.

FIG. 13 is a graph illustrating the relationship among the outerdiameter of the second magnet 19, attraction force, and driving force inthe electromagnetic transducer 5000 according to Example 5 of thepresent invention.

FIG. 14 is a cross-sectional view of an electromagnetic transducer 6000according to Example 6 of the present invention.

FIG. 15 is a magnetic flux vector diagram of the electromagnetictransducer 6000 according to Example 6 of the present invention.

FIG. 16A is a cross-sectional view of an electromagnetic transducer 7000according to Example 7 of the present invention.

FIGS. 16B and 16C are plan views of a second thin magnetic plate in theelectromagnetic transducer 7000 according to Example 7 of the presentinvention.

FIG. 17 is a partially-cutaway perspective view of a portablecommunication device incorporating an electromagnetic transduceraccording to the present invention.

FIG. 18A is a plan view of a conventional electromagnetic transducer.

FIG. 18B is a cross-sectional view of a conventional electromagnetictransducer.

FIG. 19 illustrates the relationship between a force-displacement curveof a first diaphragm and the attraction force generated by a magnet on asecond diaphragm 101 in an electromagnetic transducer.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described by way ofillustrative examples, with reference to the accompanying figures.

EXAMPLE 1

An electromagnetic transducer 1000 according to Example 1 of the presentinvention will be described with reference to FIGS. 1A, 1B, and 2.

FIGS. 1A and 1B are a plan view and a cross-sectional view,respectively, illustrating the electromagnetic transducer 1000 accordingto Example 1 of the present invention.

FIG. 2 is a magnetic flux vector diagram of the electromagnetictransducer 1000 according to Example 1 of the present invention. Themagnetic flux vector diagram of FIG. 2 only illustrates one of the twohalves with respect to a central axis (shown at the left of the figure).

As shown in FIG. 1B, the electromagnetic transducer 1000 according toExample 1 of the present invention includes a cylindrical first housing7 and a disk-shaped yoke 6 disposed so as to cover the bottom face ofthe first housing 7. A centerpole 3, which may form an integral part ofthe yoke 6, is provided in a central portion of the yoke 6. A coil 4 iswound around the center pole 3. Spaced from the outer periphery of thecoil 4 is provided an annular first magnet 5, with an appropriateinterspace maintained between the coil 4 and the inner periphery of theannular first magnet 5 around the entire circumference thereof. Anappropriate interspace is maintained between the outer peripheralsurface of the first magnet 5 and the inner peripheral surface of thefirst housing 7 around the entire circumference thereof. An upper end ofthe first housing 7 supports a first diaphragm 1, which is made of amagnetic disk, in a manner to allow vibration of the first diaphragm 1.An appropriate interspace exists between the first diaphragm 1 and thecoil 4, and between the first diaphragm 1 and the center pole 3. In acentral portion of the first diaphragm 1, a second diaphragm 2 which ismade of a magnetic disk is provided so as to be concentric with thefirst diaphragm 1. A cylindrical second housing 10 is provided so as tocover the upper face of the first housing 7. A second magnet 9 isprovided on the second housing 10 so as to be located above the seconddiaphragm 2. The second magnet 9 has a disk shape as shown in FIG. 1C.The first diaphragm 1 may be, for example, adhered to the first housing7. Alternatively, the first diaphragm 1 may be affixed by beinginterposed between the first housing 7 and the second housing 10.

As shown in FIG. 1A, a plurality of air holes 12 are formed in thesecond housing 10 for allowing the sound generated from the firstdiaphragm 1 and the second diaphragm 2 to be emitted to the exterior.The second housing 10 also serves as a cover for protecting the firstand second diaphragms 1 and 2 from external impacts. In the yoke 6, aplurality of air holes 8 are formed at predetermined intervals along thecircumferential direction for allowing the space between the coil 4 andthe inner peripheral surface of the first magnet 5 to communicate withthe exterior space lying outside the space between the first diaphragm 1and the yoke 6. Each air hole 8 allows the air to be released to theexterior so as to reduce the acoustic load on the first diaphragm 1.

Next, the operation and effects of the above-described electromagnetictransducer 1000 will be described.

In an initial state where no current flows through the coil 4, as shownin FIG. 2, a first magnetic path is formed by the first magnet 5, thefirst diaphragm 1, the second diaphragm 2, the center pole 3, and theyoke 6. A second magnetic path is formed by the second magnet 9 and thesecond diaphragm 2.

In this configuration, a downward attraction force generated by thefirst magnetic path and an upward attraction force generated by thesecond magnetic path cancel each other in relation to the seconddiaphragm 2. As a result, the first diaphragm 1 is hardly displaced bythe downward attraction force generated by the first magnetic path.

If an alternating current flows through the coil 4 in this initialstate, an alternating magnetic field is generated so that a drivingforce is generated on the second diaphragm 2. Such driving forcegenerated on the second diaphragm 2 causes the second diaphragm 2 tovibrate from its initial state, along with the fixed first diaphragm 1,due to interaction with the attraction force which is generated by thefirst magnet 5. This vibration is transmitted as sound.

In this case, the first diaphragm 1 is hardly displaced by the downwardattraction force generated by the first magnetic path. Therefore, theresonance frequency depends on an elastic constant in the neighborhoodof the origin on the force-displacement curve of the first diaphragmshown in FIG. 19. Thus, the electromagnetic transducer 1000 according tothe present example has a smaller elastic constant than in the casewhere there is an initial deflection as in the case of the conventionalelectromagnetic transducer 200, thereby resulting in a low resonancefrequency. For example, in the case of an electromagnetic transducerhaving a diameter of about 15 mm, where the first diaphragm 1 and thesecond diaphragm 2 are each formed of a permalloy and are about 30 μmthick and about 150 μm thick, respectively, the resonance frequency canbe lowered to about 1.6 kHz to 1 kHz due to the provision of the secondmagnet 9.

FIG. 3 illustrates the relationship among the outer diameter of thesecond magnet 2, attraction force, and driving force. The vertical axisrepresents the attraction force (solid line) and the driving force(broken line), whereas the horizontal axis represents the outer diameterof the second magnet 2. A negative attraction force value indicates thatthe second diaphragm 2 is being attracted toward the second magnet 9. Itis assumed that the second diaphragm 2 according to the present examplehas a diameter of about 4 mm.

As shown in FIG. 3, the attraction force becomes substantially zero whenthe outer diameter of the second magnet 9 substantially equals the outerdiameter of the second diaphragm 2, so that the upward and downwardattraction forces which act on the second diaphragm 2 are atequilibrium. As the outer diameter of the second magnet 9 increases fromthis value, the second diaphragm 2 is attracted more strongly toward thecenter pole 3, despite the increase in the volumetric size of the secondmagnet 9. On the other hand, as the outer diameter of the second magnet9 decreases, the second diaphragm 2 is attracted more toward the secondmagnet 9. From these results, it will be seen that the second diaphragm2 is attracted more strongly toward the second magnet 9 as the outerdiameter of the second magnet 9 decreases.

These results show that, as the outer diameter of the second magnet 9 isdecreased, the second diaphragm 2 may be attracted too strongly towardthe second magnet 9 at certain diameters of the second magnet 9. In suchcases, the attraction force can be adjusted by replacing the secondmagnet 9 with a magnet having a smaller thickness or a smaller energyproduct. By replacing the second magnet 9 with a magnet having a smallerthickness or a smaller energy product, it becomes possible to reduce thesize of the electromagnetic transducer 1000 and the leakage flux towardthe exterior of the electromagnetic transducer 1000 can be reduced.

As described above, it is preferable that the outer diameter of thesecond magnet 9 is equal to or smaller than the outer diameter of thesecond diaphragm 2.

Although the magnetization direction of the second magnet 9 isillustrated as being in the same direction as that of the first magnet 5according to the present example, it is also possible to magnetize thesecond magnet 9 and the first magnet 5 in opposite directions.

EXAMPLE 2

An electromagnetic transducer 2000 according to Example 2 of the presentinvention will be described with reference to FIG. 4.

FIG. 4 is a cross-sectional view of the electromagnetic transducer 2000according to Example 2 of the present invention.

In accordance with the electromagnetic transducer 2000 shown in FIG. 4,a third magnet 11 is provided, e.g., by being adhered, to the seconddiaphragm 2. A first magnet 405 and a second magnet 409 provide the sameeffects as those provided by the first magnet 5 and the second magnet 9,respectively, described in Example 1. However, the respective energyproducts of the first magnet 405 and a second magnet 409 are adjusted sothat appropriate magnetic paths are formed between themselves and thethird magnet 11. Otherwise the electromagnetic transducer 2000 has thesame structure as that of the electromagnetic transducer 1000 accordingto Example 1. The magnetization direction of the third magnet 11 isopposite to that of the first magnet 405 and the second magnet 409.

The operation of the electromagnetic transducer 2000 according toExample 2 is similar to that of the electromagnetic transducer 1000according to Example 1 except that the third magnet 11 is present on thesecond diaphragm 2. Since the third magnet 11 is magnetized in theopposite direction to that of the first magnet 405 and the second magnet409, it is possible to prevent the first diaphragm 1 or the seconddiaphragm 2 from being attracted onto the first magnet 405 or the secondmagnet 409 when the first diaphragm 1 deflects or vibrates.

As a result, a highly-durable electromagnetic transducer can be providedsuch that even when the elastic force of the first diaphragm 1 haschanged after a long period of use of the electromagnetic transducer,the first diaphragm 1 or the second diaphragm 2 is prevented from beingattracted onto the first magnet 405 or the second magnet 409.

Although the third magnet 11 is illustrated as being provided on thesecond diaphragm 2, the third magnet 11 may be provided in the center ofthe lower face of the first diaphragm 1. Alternatively, third magnets 11may be provided in the center of the upper face and the lower face ofthe first diaphragm 1.

EXAMPLE 3

An electromagnetic transducer 3000 according to Example 3 of the presentinvention will be described with reference to FIGS. 5 and 6.

FIGS. 5 and 6 are a cross-sectional view and a magnetic flux vectordiagram, respectively, of the electromagnetic transducer 3000 accordingto Example 3 of the present invention. The magnetic flux vector diagramof FIG. 6 only illustrates one of the two halves with respect to acentral axis (shown at the left of the figure).

In accordance with the electromagnetic transducer 3000, a second magnet29 is supported by a second housing 10 so that the second magnet 29 islocated above the second diaphragm 2. The second magnet 29 is magnetizedalong a radial direction with respect to an axis through the center ofthe second diaphragm 2. Otherwise, the electromagnetic transducer 3000has the same structure as that of the electromagnetic transducer 1000according to Example 1.

In accordance with the electromagnetic transducer 3000 of Example 3, afirst magnetic path is formed by a first magnet 5, a first diaphragm 1,the second diaphragm 2, a center pole 3, and a yoke 6. A second magneticpath is formed by the second magnet 29 and the second diaphragm 2. Theformation of the first and second magnetic paths is based on the sameprinciple as that for the electromagnetic transducer 1000 according toExample 1. The operation of the electromagnetic transducer 3000according to Example 3 is basically similar to that of theelectromagnetic transducer 1000 according to Example 1.

One difference from Example 1 is the magnetization direction of thesecond magnet 29. As shown in FIG. 6, the second magnet 29 is radiallymagnetized in the opposite direction to the direction of the magneticflux vector on the second diaphragm 2, so that the magnetic paths can beformed more efficiently. As a result, the leakage flux is reduced ascompared to that in Example 1 (see the magnetic flux vector diagram ofFIG. 2).

Since the magnetic paths can be formed more efficiently, it is possibleto reduce the thickness of the second magnet 29. For example, in thecase where a radially magnetized ferrite magnet is used as the secondmagnet 29, the thickness of the second magnet 29 which is required inorder to obtain similar effects to those attained by Example 1 will beabout ⅓ of the thickness of the second magnet 9 according to Example 1.

Although ferrite is illustrated as a material for the second magnet 29,it is also possible to employ neodymium or the like in order to furtherreduce the thickness of the second magnet 29. It is also possible toemploy samarium cobalt for the second magnet 29 in order to obtain goodheat resistance.

EXAMPLE 4

An electromagnetic transducer 4000 according to Example 4 of the presentinvention will be described with reference to FIGS. 7 and 8.

FIG. 7 is a cross-sectional view of the electromagnetic transducer 4000according to Example 4 of the present invention.

In accordance with the electromagnetic transducer 4000 as shown in FIG.7, a first diaphragm 31, which is made of a non-magnetic material (e.g.,titanium), is affixed by being interposed between a first housing 7 anda second housing 10. The first diaphragm 31 has the shape of a disk suchthat a portion of the disk is elevated along a direction perpendicularto the radial direction of the disk. A first magnet 705 and a secondmagnet 709 provide the same effects as those of the first magnet 5 andthe second magnet 9, respectively, described in Example 1. However,since the first diaphragm 31 is non-magnetic, the respective energyproducts of a first magnet 705 and a second magnet 709 are adjusted sothat appropriate magnetic paths are formed. Otherwise, theelectromagnetic transducer 4000 has the same structure as that of theelectromagnetic transducer 1000 according to Example 1.

The operation and effects of the electromagnetic transducer 4000 havingthe above-described structure will be described. The operation of theelectromagnetic transducer 4000 according to Example 4 is basicallysimilar to that of the electromagnetic transducer 1000 according toExample 1.

In the case where the first diaphragm 31 is made of a non-magneticmaterial, the attraction force and the driving force which are generatedon the second diaphragm 2 are constant regardless of the shape of thefirst diaphragm 31.

In general, when a sine-wave current is input to a coil 4, the drivingforce generated on the second diaphragm 2 does not necessarily appear asa sine wave having the same amplitude on the plus side (i.e., in thedirection in which the diaphragm goes away from a magnetic circuit) andthe negative side (i.e., in the direction in which the diaphragm comestoward the magnetic circuit). For example, the ratio between the plusside and the minus side may be about 0.85:1.00, so that the drivingforce is biased toward the minus side. Such non-linearity may causeharmonic distortion.

Therefore, according to Example 4, the shape of the first diaphragm 31is designed so that the force-displacement characteristics of the firstdiaphragm 31 define an inverse of the biased driving force generated onthe second diaphragm 2, thereby canceling the non-linearity of thedriving force.

FIG. 8 shows the force-displacement curve of the first diaphragm 31shown in FIG. 7. The first diaphragm 31 is shaped so as to havedifferent elastic constants depending on whether to be deformed towardthe plus side or the negative side, i.e., the force-displacement curveof the first diaphragm 31 defines an inverse of the aforementionedbiased driving force generated on the second diaphragm 2. As a result,the entire system which combines the driving force and the elasticity ofthe first diaphragm 31 provides a substantially linearforce-displacement curve for the first diaphragm 31, thereby enablingsound reproduction at a low distortion level.

Although the first diaphragm 31 is illustrated as being shaped so that aportion of the disk is elevated along a direction perpendicular to theradial direction of the disk, any shape that can realize inversecharacteristics of the driving characteristics can be adopted for thefirst diaphragm 31. For example, a portion of the first diaphragm 31 maybe undulated.

Although the first diaphragm 31 is illustrated as being non-magnetic inorder to facilitate the designing of the electromagnetic transducer4000, it is also possible to employ a magnetic material for the firstdiaphragm 31 for an increased driving force. Although the illustratedfirst diaphragm 31 is affixed by being interposed between the firsthousing 7 and the second housing 10, the first diaphragm 31 mayalternatively be affixed via adhesion.

EXAMPLE 5

An electromagnetic transducer 5000 according to Example 5 of the presentinvention will be described with reference to FIGS. 9A, 9B, and 10.

FIGS. 9A and 10 are a cross-sectional view and a magnetic flux vectordiagram, respectively, of the electromagnetic transducer 5000 accordingto Example 5 of the present invention. The magnetic flux vector diagramof FIG. 10 only illustrates one of the two halves with respect to acentral axis (shown at the left of the figure) of the electromagnetictransducer 5000.

In accordance with the electromagnetic transducer 5000 as shown in FIG.9A, a first diaphragm 41, which is made of a non-magnetic material, isaffixed by being interposed between a first housing 7 and a secondhousing 10. The first diaphragm 41 has the shape of a disk such that aportion of the disk is elevated along a direction perpendicular to theradial direction of the disk. In a central portion of the firstdiaphragm 41, a second diaphragm 22 which is made of a magnetic disk isprovided so as to be concentric with the first diaphragm 41.Furthermore, an annular second magnet 19 as shown in FIG. 9B is providedon the second housing 10 so as to be located above the second diaphragm22. An annular thin magnetic plate 13 is provided on a face of the firstmagnet 905 opposing the first diaphragm 41. On the inner peripheralsurface of a first magnet 905, a concave portion for receiving the thinmagnetic plate 13 is provided.

According to the present example, the first diaphragm 41 is made oftitanium, which is a non-magnetic material, and has a thickness of about15 μm; and the second diaphragm 22 is made of a permalloy and has athickness of about 50 μm. Such a thickness of the second diaphragm 22allows a magnetic saturation to occur when the first diaphragm 41 isdeflected toward the center pole 3. The second magnet 19 is magnetizedalong the height direction thereof, as is the first magnet 905.Otherwise, the electromagnetic transducer 5000 has the same structure asthat of the electromagnetic transducer 4000 according to Example 4 asshown in FIG. 7.

The operation and effects of the electromagnetic transducer 5000 havingthe above-described structure will be described.

In an initial state where no current flows through the coil 4, as shownin FIG. 10, a first magnetic path is formed by the first magnet 905, thethin magnetic plate 13, the second diaphragm 22, the center pole 3, andthe yoke 6. A second magnetic path is formed by the second magnet 19 andthe second diaphragm 22.

The provision of the thin magnetic plate 13 as shown in FIG. 9A makes itpossible to efficiently flow an alternating magnet flux through thesecond diaphragm 22, whereby the driving force is increased. As aresult, the reproduced sound pressure level is increased.

Since the first diaphragm 41 is made of non-magnetic titanium accordingto the present example, the first diaphragm 41 is omitted from themagnetic flux vector diagram shown in FIG. 10.

FIG. 11 shows the attraction force generated on the second diaphragm 22in the case where the second magnet 19 is provided (solid line: presentexample) and in the case where the second magnet 19 is not provided(broken line: conventional). The vertical axis represents the attractionforce, whereas the horizontal axis represents the distance from a “zeropoint” of the second diaphragm 22. As used herein, the “zero point” isdefined as a position of the second diaphragm 22 at which a downwardattraction force generated by the first magnet 905 and an upwardattraction force generated by the second magnet 19, both acting on thesecond diaphragm 22, are at equilibrium.

As seen from FIG. 11, in the case where the second magnet 19 is notprovided (broken line), the attraction force always takes a positivevalue because the second diaphragm 22 is attracted to the first magnet905.

On the other hand, in the case where the second magnet 19 is provided(solid line), a attraction force is generated in the opposite directionfrom the center pole 3 as well. Therefore, the attraction force takesboth positive values and negative values with respect to the zero pointat which both attraction forces on the second diaphragm 22 are atequilibrium.

The second diaphragm 22 is relatively thin, e.g., about 50 μm, therebyfacilitating magnetic saturation. The attraction force is prevented fromdrastically increasing toward the center pole 3, as in the case of aconventional electromagnetic transducer.

Based on this structure, the attraction force exhibits substantiallylinear characteristics with respect to the distance from the zero point,as shown in FIG. 11. As a result, the stiffness of the entire system,which is determined based on the difference between the elastic force ofthe first diaphragm 41 and the attraction force acting on the seconddiaphragm 19, can be reduced and the resonance frequency, which isdetermined based on the stiffness, can also be reduced.

The stiffness of the entire system is constant independent of thedistance so long as the first diaphragm 41 has a linear elastic force.Therefore, the resonance frequency does not change due to rises andfalls of an applied voltage. Thus, the harmonic distortion is minimized.

FIG. 12 shows the driving forces generated on the second diaphragm 22 inthe case where the second magnet 19 is provided (solid line: presentexample) and in the case where the second magnet 19 is not provided(broken line: conventional). The vertical axis represents the drivingforce, whereas the horizontal axis represents the distance from thecenter pole 3.

As seen from FIG. 12, in the case where the second magnet 19 is omitted,magnetic saturation occurs because of the use of the thin seconddiaphragm 22, and sufficient driving force cannot be obtained.

Therefore, the second magnet 19 is added so as to cancel the magneticflux generated on the second diaphragm 22 by the first magnet 905,thereby alleviating magnetic saturation. As a result, the alternatingmagnetic flux which provides the driving force is allowed to efficientlyflow through the second diaphragm 22, thereby increasing the resultantdriving force. In other words, according to the present example, it ispossible to obtain a sufficient driving force even when a thin diaphragmis used, although such a diaphragm is likely to cause magneticsaturation. The use of a thin diaphragm reduces the mass of thevibrating system, resulting in a further increase in the reproducedsound pressure level.

FIG. 13 illustrates the relationship among the outer diameter of thesecond magnet 19, attraction force, and driving force. The vertical axisrepresents the attraction force (solid line) and the driving force(broken line), whereas the horizontal axis represents the outer diameterof the second magnet 19. A larger attraction force value indicates thatthe second diaphragm 22 is being attracted more toward the center pole3. It is assumed that the second diaphragm 22 according to the presentexample has a diameter of about 4 mm.

As seen from FIG. 13, the change in the attraction force is relativelysmall when the outer diameter of the second magnet 19 is smaller thanthe outer diameter of the second diaphragm 22. However, as the outerdiameter of the second magnet 19 exceeds about 4 mm (at which the outerdiameter of the second magnet 19 equals the outer diameter of the seconddiaphragm 22), the change in the attraction force increases, and theattraction forces become closer to the zero point, or a point ofequilibrium.

From these results, it can be seen that, in the range shown in FIG. 13,it becomes easier for the attraction forces acting on the seconddiaphragm 22, i.e., the force toward the second magnet 19 and the forcetoward the center pole 3, to establish equilibrium.

On the other hand, the driving force becomes maximum when the outerdiameter of the second diaphragm 22 is about 4.5 mm (although thedifference is very small), no substantial change in the driving force isobserved responsive to the change in the outer diameter of the secondmagnet 19.

Therefore, it is preferable that the outer diameter of the second magnet19 is equal to or greater than the outer diameter of the seconddiaphragm 22.

The illustrated first diaphragm 41 is formed of non-magnetic titaniumbecause it makes for greater designing flexibility due to heatresistance and the absence of heat resistance magnetic field effects.However, it is also possible to employ a permalloy for the firstdiaphragm 41 as well as for the second diaphragm 22. In this case, sincethe first diaphragm 41 and the second diaphragm 22 are made of the samematerial, it is easy to join the two diaphragms. It is also possible touse a non-metal material, e.g., a resin, for the first diaphragm 41,whereby it becomes easy to work the first diaphragm 41 into a desiredshape.

Although the thickness of the second diaphragm 22 according to thepresent example is relatively thin, e.g., about 50 μm, so as tofacilitate magnetic saturation, the second diaphragm 22 may have a largethickness in the case where magnetic saturation is irrelevant as in thecase of Example 1. In this case, a decrease in the driving force due tosaturation in the neighborhood of the center pole 3 as shown in FIG. 12does not occur. This provides certain advantages in designs such thatthe second diaphragm 22 is deployed relatively close to the center pole3. Similar effects may also be obtained by forming the second diaphragm22 from pure iron.

Although the thin magnetic plate 13 is provided on the first magnet 905according to the present example, the thin magnetic plate 13 does notneed to be provided in the case where a sufficient driving force can beobtained with the first magnet 905 alone, or where there is notsufficient space.

According to the present example, the thickness of the second diaphragm22 is made relatively thin to cause magnetic saturation in order toensure that the attraction forces generated by the magnetic path formedby the first magnet 905, the center pole 3, and the yoke 6 and thesecond magnet 19 are substantially linear with respect to the distancefrom the center pole 3. However, other measures can also be taken solong as similar effects are attained. For example, it can be ensuredthat the aforementioned attraction forces are substantially linear withrespect to the distance from the center pole 3 by adjusting the shape ofthe second diaphragm 22, e.g., by forming a notch or a hole in thesecond diaphragm 22.

EXAMPLE 6

An electromagnetic transducer 6000 according to Example 6 of the presentinvention will be described with reference to FIGS. 14 and 15.

FIGS. 14 and 15 are a cross-sectional view and a magnetic flux vectordiagram, respectively, of the electromagnetic transducer 6000 accordingto Example 6 of the present invention. The magnetic flux vector diagramof FIG. 15 only illustrates one of the two halves with respect to acentral axis (shown at the left of the figure) of the electromagnetictransducer 6000.

In accordance with the electromagnetic transducer 6000 as shown in FIG.14, an annular second magnet 39 which is provided on a second housing 10is magnetized along a radial direction with respect to an axis throughthe center of a second diaphragm 22. Otherwise, the electromagnetictransducer 6000 has the same structure as that of the electromagnetictransducer 5000 according to Example 5.

In accordance with the electromagnetic transducer 6000 of Example 6, inan initial state where no current flows through the coil 4, as shown inFIG. 15, a first magnetic path is formed by a first magnet 905, a thinmagnetic plate 13, the second diaphragm 22, a centerpole 3, and a yoke6, whereas a second magnetic path is formed by a second magnet 39 andthe second diaphragm 22, as in the case of Example 5. The operation ofthe electromagnetic transducer 6000 according to Example 6 is similar tothat of the electromagnetic transducer 5000 according to Example 5.

One difference from Example 5 is the magnetization direction of thesecond magnet 39. As shown in FIG. 15, the second magnet 39 is radiallymagnetized in the opposite direction to the direction of the magneticflux vector on the second diaphragm 22, so that the magnetic paths canbe formed more efficiently. As a result, the leakage flux is reduced ascompared to that in Example 5 (see the magnetic flux vector diagram ofFIG. 10).

Since the magnetic paths can be formed more efficiently, it is possibleto reduce the thickness of the second magnet 39. For example, in thecase where a radially magnetized ferrite magnet is used as the secondmagnet 39, the thickness of the second magnet 39 which is required inorder to obtain similar effects to those attained by Example 5 will beabout two-thirds of the thickness of the second magnet 19 according toExample 5.

Although ferrite is illustrated as a material for the second magnet 39,it is also possible to employ neodymium or the like in order to furtherreduce the thickness of the second magnet 39. It is also possible toemploy samarium cobalt for the second magnet 39 in order to obtain goodheat resistance.

EXAMPLE 7

An electromagnetic transducer 7000 according to Example 7 of the presentinvention will be described with reference to FIGS. 16A and 16B.

FIG. 16A is a cross-sectional view of the electromagnetic transducer7000 according to Example 7 of the present invention.

In accordance with the electromagnetic transducer 7000 as shown in FIG.16A, an annular second thin magnetic plate 33 as shown in FIG. 16B isprovided on the upper face of a second magnet 619. In a second housing610, a concave portion for receiving the second thin magnetic plate 33is additionally provided. In the second housing 610, a plurality of airholes for allowing the sound generated from a first diaphragm 41 and asecond diaphragm 22 to be emitted to the exterior space of the secondhousing 610. Since the second thin magnetic plate 33 is provided on theupper face of the second magnet 619, a magnetic path is formed by thesecond magnet 619, the second thin magnetic plate 33, and the seconddiaphragm 22. A first magnet 605 and the second magnet 619 provide thesame effects as those of the first magnet 905 and the second magnet 19,respectively, described in Example 5. However, since the magnetic fluxfrom the second magnet 619 is to be introduced through the second thinmagnetic plate 33, the respective energy products of the first magnet605 and the second magnet 619 are adjusted so that appropriate magneticpaths are formed. Otherwise, the electromagnetic transducer 7000 has thesame structure as that of the electromagnetic transducer 5000 accordingto Example 5.

By providing the second thin magnetic plate 33 as shown in FIG. 16, themagnetic flux of the second magnet 619 is directed through the secondthin magnetic plate 33, so that the magnetic resistance in theaforementioned magnetic path is reduced. As a result, the energy productof the second magnet 619 can be reduced as compared to the case wherethe second thin magnetic plate 33 is omitted. In addition, since themagnetic flux from the second magnet 619 is introduced into the secondthin magnetic plate 33, the leakage magnetic flux to the exterior of theelectromagnetic transducer 7000 can be reduced.

Although the second thin magnetic plate 33 has an annular shape as shownin FIG. 16A, it is also possible to provide a disk-shaped second thinmagnetic plate 34 on the upper face of the second magnet 619 as shown inFIG. 16C.

The second thin magnetic plate 33 or 34 may also be provided on thedisk-shaped second magnet described in Examples 1 to 4 of the presentinvention.

According to the present example, the same attraction force that isprovided by a second magnet 19 which has an energy product of about 26MGOe and a thickness of about 0.7 mm but which does not have a secondthin magnetic plate 33 provided thereon (e.g., Example 5 of the presentinvention) can be attained by a second magnet 619 which has an energyproduct of about 22 MGOe and a thickness of about 0.5 mm owing to theprovision of the second thin magnetic plate 33.

FIG. 17 is a partially-cutaway perspective view of a cellular phone 61as an example of a portable communication device incorporating anelectromagnetic transducer 64 according to the present invention. Anyone of the electromagnetic transducers 1000 to 7000 according toExamples 1 to 7 of the present invention can be used as theelectromagnetic transducer 64.

The cellular phone 61 has a housing 62. A sound hole 63 is provided onone face of the housing 62. The electromagnetic transducer 64 isprovided so that a first diaphragm thereof opposes the sound hole 63.The cellular phone 61 internalizes a signal processing circuit (notshown) for receiving a call signal, converting the call signal, andinputting the converted signal to the electromagnetic transducer 64.When the signal processing circuit receives a signal indicating anincoming call, the received signal is input to the electromagnetictransducer 64, whereby the electromagnetic transducer 64 reproduces aring sound to inform the user of a received call. Subsequently, an audiosignal is input to the electromagnetic transducer 64, whereby theelectromagnetic transducer 64 reproduces audio sounds so that the usercan begin talking on the phone.

Many conventional electromagnetic transducers which are internalized inportable communication devices such as cellular phones have a highresonance frequency, and are used only for reproducing a ring sound.

On the other hand, the electromagnetic transducer according to thepresent invention can have a relatively low resonance frequency. Whenused for a portable communication device, the electromagnetic transduceraccording to the present invention can also reproduce audio signals, sothat it is possible to reproduce a ring sound and audio signals by usingonly one electromagnetic transducer. As a result, the number of elementsinternalized in a cellular phone that are related to audio functions,which are conventionally provided in pluralities, can be reduced.

In the illustrated cellular phone 61, the electromagnetic transducer 64is mounted directly on the housing 62. However, the electromagnetictransducer 64 may be mounted on a circuit board which is internalized inthe cellular phone 61. An acoustic port for increasing the soundpressure level of the ring sound may be added.

Although a cellular phone is illustrated in FIG. 17 as a portablecommunication device, the present invention is applicable to anyportable communication device that requires an electromagnetictransducer which is capable of reproducing a sound at a high level in asmall-sized configuration, e.g., a pager, a notebook-type personalcomputer, or a watch.

According to Examples 1 to 7, a housing 10 or 610 for supporting thesecond magnet 9, 409, 29, 709, 19, 39, or 619 is provided. However, inthe case where the electromagnetic transducer according to any ofExamples 1 to 7 is mounted on the cellular phone 61 shown in FIG. 17,for example, it is possible to embed the second magnet 9, 409, 29, 709,19, 39, or 619 in the housing 62 of the cellular phone, so that thehousing 10 or 610 and the housing 62 of the cellular phone 61 can beintegrated as one piece.

INDUSTRIAL APPLICABILITY

In accordance with the electromagnetic transducer of the presentinvention, a second magnet is provided above a second diaphragm with aninterspace therebetween so that a first diaphragm can be retained in astate of equilibrium.

As a result, it is possible to decrease the resonance frequency withoutchanging any other components, thereby enabling the reproduction oflow-frequency ranges. Since the driving force upon the second diaphragmis increased and substantially linear attraction force-displacementcharacteristics are attained, it is possible to reproduce a sound at ahigh level and low distortion, without changing any other components.

Alternatively, in accordance with the electromagnetic transducer of thepresent invention, the second magnet may be magnetized along a radialdirection so that the second magnet can operate efficiently, whereby itbecomes possible to reduce the size of the second magnet.

Alternatively, in accordance with the electromagnetic transducer of thepresent invention, the first diaphragm may have non-linearity forcanceling the non-linearity of the driving force generated on the seconddiaphragm. As a result, the non-linearity of the entire system and hencethe harmonic distortion can be minimized.

Alternatively, in accordance with the electromagnetic transducer of thepresent invention, a third magnet can be provided on at least one of anupper face and a lower face of the first and second diaphragms. As aresult, the first and second diaphragms can be prevented from beingattracted onto a center pole or the second magnet.

Alternatively, in accordance with the electromagnetic transducer of thepresent invention, the second diaphragm may have a thickness whichallows a magnetic saturation to occur when the second diaphragm isdeflected toward the center pole. Thus, magnetic saturation isfacilitated, thereby controlling the attraction force which tends to beincreased as the second diaphragm moves toward the center pole. Sincemore linear static attraction characteristics are realized by this, itis possible to lower the resonance frequency.

Alternatively, in accordance with the electromagnetic transducer of thepresent invention, a thin magnetic plate may be provided on a face ofthe first magnet opposing the first diaphragm. As a result, analternating magnetic flux is efficiently allowed to flow through thesecond diaphragm, which provides an increased driving force and hence anincreased sound pressure level.

In accordance with a portable communication device according to thepresent invention incorporating the electromagnetic transducer accordingto the present invention, it is possible to reproduce alarm sounds,audio sounds, and the like on the electromagnetic transducer.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. An electromagnetic transducer comprising: a firstdiaphragm; a second diaphragm disposed in a central portion of the firstdiaphragm, the second diaphragm being made of a magnetic material; ayoke disposed so as to oppose the first diaphragm; a center poledisposed between the yoke and the first diaphragm; a coil disposed so asto surround the center pole; a first magnet disposed so as to surroundthe coil; and a second magnet disposed on an opposite side of the firstdiaphragm from the center pole.
 2. An electromagnetic transduceraccording to claim 1, further comprising: a first housing for supportingthe first diaphragm; and a second housing in which the second magnet isdisposed.
 3. An electromagnetic transducer according to claim 2, whereinthe first diaphragm is affixed by being adhered to the first housing. 4.A portable communication device comprising the electromagnetictransducer according to claim
 3. 5. An electromagnetic transduceraccording to claim 2, wherein the first diaphragm is affixed by beinginterposed between the first housing and the second housing.
 6. Aportable communication device comprising the electromagnetic transduceraccording to claim
 5. 7. An electromagnetic transducer according toclaim 2, wherein the second housing is a cover for protecting the firstdiaphragm and the second diaphragm.
 8. A portable communication devicecomprising the electromagnetic transducer according to claim
 7. 9. Aportable communication device comprising the electromagnetic transduceraccording to claim
 2. 10. An electromagnetic transducer according toclaim 1, wherein the second magnet has a disk shape.
 11. Anelectromagnetic transducer according to claim 10, wherein an outerdiameter of the second magnet is equal to or smaller than an outerdiameter of the second diaphragm.
 12. A portable communication devicecomprising the electromagnetic transducer according to claim
 11. 13. Aportable communication device comprising the electromagnetic transduceraccording to claim
 10. 14. An electromagnetic transducer according toclaim 1, wherein the second magnet has an annular shape.
 15. Anelectromagnetic transducer according to claim 14, wherein an outerdiameter of the second magnet is equal to or greater than an outerdiameter of the second diaphragm.
 16. A portable communication devicecomprising the electromagnetic transducer according to claim
 15. 17. Aportable communication device comprising the electromagnetic transduceraccording to claim
 14. 18. An electromagnetic transducer according toclaim 1, further comprising a third magnet in a central portion of atleast one face of the first diaphragm or the second diaphragm.
 19. Aportable communication device comprising the electromagnetic transduceraccording to claim
 18. 20. An electromagnetic transducer according toclaim 1, wherein the second magnet is magnetized in the same directionas the first magnet.
 21. A portable communication device comprising theelectromagnetic transducer according to claim
 20. 22. An electromagnetictransducer according to claim 1, wherein the second magnet is magnetizedalong a radial direction with respect to an axis through a center of thecenter pole.
 23. A portable communication device comprising theelectromagnetic transducer according to claim
 22. 24. An electromagnetictransducer according to claim 1, wherein the second diaphragm has athickness which allows a magnetic saturation to occur when the seconddiaphragm reaches a neighborhood of an upper face of the center pole.25. A portable communication device comprising the electromagnetictransducer according to claim
 24. 26. An electromagnetic transduceraccording to claim 1, wherein the first diaphragm is made of a magneticmaterial.
 27. A portable communication device comprising theelectromagnetic transducer according to claim
 26. 28. An electromagnetictransducer according to claim 1, wherein the first diaphragm is made ofa non-magnetic material.
 29. A portable communication device comprisingthe electromagnetic transducer according to claim
 28. 30. Anelectromagnetic transducer according to claim 1, further comprising afirst magnetic plate provided between the first magnet and the firstdiaphragm.
 31. An electromagnetic transducer according to claim 30,wherein the first magnetic plate has an annular shape.
 32. A portablecommunication device comprising the electromagnetic transducer accordingto claim
 31. 33. A portable communication device comprising theelectromagnetic transducer according to claim
 30. 34. An electromagnetictransducer according to claim 1, further comprising a second magneticplate disposed on the second magnet.
 35. An electromagnetic transduceraccording to claim 34, wherein the second magnetic plate has a diskshape.
 36. A portable communication device comprising theelectromagnetic transducer according to claim
 35. 37. An electromagnetictransducer according to claim 34, wherein the second magnetic plate hasan annular shape.
 38. A portable communication device comprising theelectromagnetic transducer according to claim
 37. 39. A portablecommunication device comprising the electromagnetic transducer accordingto claim
 34. 40. An electromagnetic transducer according to claim 1,wherein the first diaphragm is shaped so as to have force-displacementcharacteristics for substantially canceling non-linearity of a drivingforce generated on the second diaphragm.
 41. A portable communicationdevice comprising the electromagnetic transducer according to claim 40.42. An electromagnetic transducer according to claim 1, wherein there isa substantially linear relationship between a distance between thesecond diaphragm and the center pole and a resultant of: a firstattraction force generated on the second diaphragm by a magnetic circuitcomprising the first magnet, the center pole, and the yoke; and a secondattraction force generated on the second diaphragm by the second magnet.43. A portable communication device comprising the electromagnetictransducer according to claim
 42. 44. A portable communication devicecomprising the electromagnetic transducer according to claim
 1. 45. Aportable communication device according to claim 44, further comprisinga third housing having a sound hole therein, wherein the electromagnetictransducer is disposed so that the first diaphragm and the seconddiaphragm oppose the sound hole.
 46. A portable communication deviceaccording to claim 45, wherein the second magnet is disposed in thethird housing.
 47. A portable communication device comprising theelectromagnetic transducer according to claim 44.